1. Field of the Invention
This invention relates to a mechanism for suppressing vibrations in travel of a mobile or travelling crane like a rough terrain crane.
2. Description of the Related Art
As shown particularly in FIG. 4, mobile cranes generally have a boom 3 rotatably supported on a vehicle body 2, which is supported on vehicle wheels, for rotation about a horizontal shaft 5 through operation of a boom lifting cylinder 3. This sort of mobile crane has a problem that the vibrations to which the vehicle body 2 is subjected during travel due to undulations on the road surfaces or abrupt accelerations or decelerations of the vehicle are augmented by vertical oscillatory movements of the boom 3 or other attachments, giving an uncomfortable feeling to the operator.
As means for suppressing such vibrations during travel, there has been known in the art an apparatus as described in Japanese Laid-Open Patent Application No. 59-18295, which is constituted, as shown in FIG. 5, by a damper mechanism 19 provided within a boom lifting hydraulic cylinder 18, a counter-balancing valve 12 provided in a conduit 13 in communication with an oil chamber 181 which holds the load of the cylinder 18, and an electromagnetic selector valve 16, a shuttle valve 15 in communication with the conduit 13, a conduit connected to the other oil chamber 182 and a conduit connected to a direction control valve 11.
According to this apparatus, if the direction control valve 11 is switched to a boom-up or boom-down position during crane operation with a selector valve 16 in position a, the discharge oil of a pump 10 is fed to the oil chamber 181 or 182 of the cylinder, thereby expanding or contracting the cylinder 18 to lift up or down the boom. For a vehicle travelling operation, the change-over valve 16 in position a is switched to position b after setting the fore end of the boom in an appropriate travel position at a height of H.sub.1 from the road surface by contracting the cylinder 18. As a result, the conduit 13 is communicated with the conduit 17 through the change-over valve 16 and the shuttle valve 15, forming a closed circuit through the oil chambers 181 and 182 and the damper mechanism 191 to introduce and accumulate, in the damper mechanism 191, the load pressure acting in the oil chamber 181 of the cylinder 18, thereby to produce the vibration damping effect while the crane is in travel.
In order to provide the damping action effectively in this sort of mobile crane, it is necessary to set the boom 3 at a suitable height H.sub.1 during travel, more specifically, at a height H.sub.1 which is higher than a lower limit H.sub.0 but lower than an upper limit H.sub.2 which is stipulated in traffic regulations with regard to travels on public roads, namely, at a height of H.sub.0 &lt;H.sub.1 &lt;H.sub.2. Besides, the load-suspending hook has to be fixed to the vehicle body while holding the hoisting rope in an appropriately loosened state.
However, in case of the above-described known apparatus, the operator has to switch the direction control valve 11 and adjust the height of the boom 3 through expansion or contraction of the cylinder 18 while watching the height of the boom 3. This is very troublesome and causes tremendous mental fatigue. In addition, the boom height adjustment is often inaccurate, resulting in insufficient vibration damping actions due to a boom position which is too low or exceeds the upper limit height H.sub.2. Further, there are possibilities of an unloosened hoisting rope restraining the oscillatory movements of the boom which are necessary for the suppression of vibrations in travel, failing to produce the expected vibration suppressing effects. Further, a hoisting rope which is in an excessively loosened state might be swung to and fro, or a hook might hit against the vehicle body, producing uncomfortable noises.